A series of long-range acoustic propagation experiments have been conducted in the North Pacific Ocean during the last 15 years using various combinations of low-frequency, wide-bandwidth transmitters and horizontal and vertical line array receivers, including a 2-dimensional array with a maximum vertical aperture of 1400 m and a horizontal aperture of 3600 m. These measurements were undertaken to further our understanding of the physics of low-frequency, broadband propagation and the effects of environmental variability on signal stability and coherence. In this volume some of the results are presented. In the present paper the central issues these experiments have addressed are briefly summarized.

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Worcester
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,
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,
W. S.
Hodgkiss
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T. F.
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This experiment has become known as the ATOC Acoustic Engineering Test (AET) and is referred to by that name in some of the papers in this volume.
3.
This experiment has become known as the Alternate Source Test (AST).
4.
A summary of many results up to 1999 is found in
J. A.
Colosi
and
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9.
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10.
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14.
This term was first used in
M. A.
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and
F. D.
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, “
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J. Acoust. Soc. Am.
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15.
Sometimes described as explosive growth of an acoustic beam.
16.
K. C. Hegewisch, N. R. Cerruti, and S. Tomsovic, “Ocean acoustic wave propagation and ray method correspondence: Internal wave fine structure,” J. Acoust. Soc. Am., this volume.
17.
I. P. Smirnov, A. L. Virovlyansky, and G. M. Zaslavsky, “Ray chaos, travel time modulation and sensitivity to the initial conditions,” J. Acoust. Soc. Am., this volume.
18.
W. H. Munk, P. F. Worcester, and C. Wunsch, Ocean Acoustic Tomography (Cambridge University Press, Cambridge, England, 1995).
19.
M. G. Brown, F. J. Beron-Vera, I. Rypina, and I. Udovydchenkov, “Rays, modes, wavefield structure and wavefield stability,” J. Acoust. Soc. Am., this volume.
20.
S. D.
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and
N. Y.
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21.
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22.
A. K. Morozov and J. A. Colosi, “Entropy and scintillation analysis of acoustical beam propagation through ocean internal waves,” J. Acoust. Soc. Am., this volume.
23.
Bottom interactions are less of an issue for the Pioneer Seamount source, because the bottom slope on Pioneer Seamount is much steeper than the slope on Kauai.
24.
K. D. Heaney, “The Kauai near-source test (KNST): Modeling and measurements of downslope propagation near the NPAL Kauai source,” J. Acoust. Soc. Am., this volume.
25.
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26.
P. F.
Worcester
,
B. D.
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,
M. A.
Dzieciuch
,
W. H.
Munk
,
B. M.
Howe
,
J. A.
Mercer
,
R. C.
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,
J. A.
Colosi
,
K.
Metzger
,
T. G.
Birdsall
, and
A. B.
Baggeroer
, “
A test of basin-scale acoustic thermometry using a large-aperture vertical array at 3250-km range in the eastern North Pacific Ocean
,”
J. Acoust. Soc. Am.
105
,
3185
3201
(
1999
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27.
J. L.
Spiesberger
and
K.
Metzger
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J. Acoust. Soc. Am.
89
,
2677
2688
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28.
B. D.
Dushaw
,
P. F.
Worcester
,
B. D.
Cornuelle
, and
B. M.
Howe
, “
On equations for the speed of sound in seawater
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93
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255
275
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29.
J. L.
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Is Del Grosso’s sound-speed algorithm correct?
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93
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2237
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30.
For a more complete discussion of the ATOC measurements see
ATOC Consortium (A. B. Baggeroer, T. G. Birdsall, C. Clark, J. A. Colosi, B. D. Cornuelle, D. Costa, B. D. Dushaw, M. Dzieciuch, A. M. G. Forbes, B. M. Howe, D. Menemenlis, J. A. Mercer, K. Metzger, W. H. Munk, R. C. Spindel, P. F. Worcester, and C. Wunsch)
, “
Ocean climate change: comparison of acoustic tomography, satellite altimetry, and modeling
,”
Science
281
,
1327
1332
(
1998
);
B. D.
Dushaw
,
B. M.
Howe
,
J. A.
Mercer
,
R. C.
Spindel
, and
the ATOC Group
, “
Multimegameter range acoustic data obtained by bottom hydrophone arrays for measurement of ocean temperature
,”
IEEE J. Ocean. Eng.
24
,
202
214
(
1999
);
B. D. Dushaw, G. Bold, C.-S. Chui, J. Colosi, B. Cornuelle, Y. Desaubies, M. Dzieciuch, A. Forbes, F. Gaillard, J. Gould, B. Howe, M. Lawrence, J. Lynch, D. Menemenlis, J. Mercer, P. Mikhaelvsky, W. Munk, I. Nakano, F. Schott, U. Send, R. Spindel, T. Terre, P. Worcester, and C. Wunsch, “Observing the ocean in the 2000’s: A strategy for the role of acoustic tomography in ocean climate observation,” in Observing the Oceans in the 21st Century, edited by C. J. Koblinsky and N. R. Smith (GODAE Project Office and Bureau of Meteorology, Melbourne, 2001), pp. 391–418.
31.
K. R.
Curtis
,
B. M.
Howe
, and
J. A.
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, “
Low-frequency ambient sound in the North Pacific: Long time series observations
,”
J. Acoust. Soc. Am.
106
,
3189
3200
(
1999
).
32.
R. K.
Andrew
,
B. M.
Howe
,
J A.
Mercer
, and
M. A.
Dzieciuch
, “
Ocean ambient sound: Comparing the 1960s with the 1990s for a receiver off the California coast
,”
ARLO
3
,
65
69
(
2002
).
33.
A. B. Baggeroer, E. K. Scheer, and the NPAL Group (J. A. Colosi, B. D. Cornuelle, B. D. Dushaw, M. A. Dzieciuch, B. M. Howe, J. A. Mercer, W. H. Munk, R. C. Spindel, and P. F. Worcester), “Statistics and vertical directionality of low frequency ambient noise at the NPAL site,” J. Acoust. Soc. Am., this volume.
34.
W. W. L.
Au
,
P. E.
Nachtigall
, and
J. L.
Pawloski
, “
Acoustic effects of the ATOC signal (75 Hz, 195 dB) on dolphins and whales
,”
J. Acoust. Soc. Am.
101
,
2973
2977
(
1997
).
35.
D. P. Costa, D. E. Crocker, D. M. Waples, P. M. Webb, J. Gedamke, D. S. Houser, P. D. Goley, B. J. Le Boeuf, and J. Calambokidis, “The California Marine Mammal Research Program of the Acoustic Thermometry of Ocean Climate Experiment,” in California and the World Ocean ’97, San Diego, California, March 24–27, 1997, edited by O. T. Magoon et al. (American Society of Civil Engineers, Reston, VA, 1998), pp. 1542–1553.
36.
A. S.
Frankel
and
C. W.
Clark
, “
Results of low-frequency playback of M-sequence noise to humpback whales, Megaptera novaeangliae, in Hawai’i
,”
Can. J. Zool.
76
,
521
535
(
1998
).
37.
A. S.
Frankel
and
C. W.
Clark
, “
Behavioral responses of humpback whales (Megaptera novaeangliae) to full-scale ATOC signals
,”
J. Acoust. Soc. Am.
108
,
1930
1937
(
2000
).
38.
A. S.
Frankel
and
C. W.
Clark
, “
ATOC and other factors affecting the distribution and abundance of humpback whales (Megaptera novaeangliae) off the north shore of Kauai
,”
Marine Mammal Sci.
18
,
644
662
(
2002
).
39.
D. P.
Costa
,
D. E.
Crocker
,
J.
Gedamke
,
P. M.
Webb
,
D. S.
Houser
,
S. B.
Blackwell
,
D.
Waples
,
S. A.
Hayes
, and
B. J. Le
Boeuf
, “
The effect of a low-frequency sound source (acoustic thermometry of the ocean climate) on the diving behavior of juvenile northern elephant seals, Mirounga angustirostris
,”
J. Acoust. Soc. Am.
113
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1155
1165
(
2003
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40.
J. R. Mobley, Jr., “Assessing responses of humpback whales to NPAL transmissions: Results of 2001–2003 aerial surveys north of Kauai,” J. Acoust. Soc. Am., this volume.
41.
J. C.
Burtenshaw
,
E. M.
Oleson
,
J. A.
Hildebrand
,
M. A.
McDonald
,
R. K.
Andrew
,
B. M.
Howe
, and
J. A.
Mercer
, “
Acoustic and satellite remote sensing of blue whale seasonality and habitat in the northeast Pacific
,”
Deep-Sea Res., Part II
51
,
967
986
(
2004
).
42.
The modulation angle, φ=tan−1√1023=88.21°, is chosen to achieve a spectrum with a smooth sin x/x envelope.
43.
The Kauai source transmitted from 30 October 1997 to 3 October 1999, when the permit for its operation ended. The final authorization needed to resume transmissions was received on 23 January 2002, and the source transmissions started on 24 January 2002 for a 5-year period.
44.
A matched filter produces the maximum signal-to-noise output if the input signal is known exactly, and the background noise is white.
45.
M. A. Dzieciuch (private communication, 2002).
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